1. Field of the Invention
The present invention relates to the field of etchant solution, and in particular to a potentiometric titration method for measuring concentration of acid mixture of aluminum etchant.
2. The Related Arts
Wet etching processes are core processes of applying acid corrosive solutions to patternize metal layers in order to form a gate terminal, a source-drain terminal, and a pixel electrode in the fabrication of a thin-film transistor (TFT). Aluminum and molybdenum are conductive materials that commonly used to form the gate terminal and an acid etchant used may be composed of a variety of different acids, but in most cases, a mixture of strong acids (such as phosphoric acid, nitric acid, and glacial acetic acid) is applied to dissolve and oxidize/reduce aluminum and molybdenum so as to realize patternization of the gate layer.
A composition of acid mixture for aluminum etchants generally comprise phosphoric acid (70%-72%), nitric acid (1.8%-2.0%), and glacial acetic acid (8.0%-10%), among which nitric acid functions to provide H3O+ to oxidize metal aluminum for effecting wet etching. Phosphate provides phosphate group to form a complex with oxidized metal so as to dissolve metal oxides. Glacial acetic acid adheres to a surface of reactants to reduce viscosity of the etchant for increasing permeability thereof and adjusting etching rate. Therefore, well controlling concentrations of various acids contained in the etchant solution is vital to the adjustment of etching rate and formation of etched configuration.
Based on the acid-base proton theory, the acidic or basic strength that a substance exhibits in a solution is not only pertaining to the nature of the substance but also pertaining to the property of the solution. Measurement of acidic components in an aqueous solution through titration can only be done when Pka between different acids is as high as 5. Thus, it is generally impossible to distinguish nitric acid (of which PKa is −1.32) from the first dissociation of phosphoric acid (of which PKa1 is 1.96) or to distinguish acetic acid (of which PKa is 4.73) from the second dissociation of phosphoric acid (of which PKa2 is 7.12). Due to the so called “leveling effect” of aqueous solutions, distinction of various acids in an acid mixture can only be done with a multiple-stage process or be done in a non-aqueous solution.
A potentiometric titration method determines the titration end point through detecting an abrupt potential change during titrating. This method shows high sensitivity and accuracy and may realize automatic and successive titration and is thus of wide applications. Existing approaches of acid mixture in potentiometric titration are generally effected with a two-stage process applied in combination with appropriate non-aqueous solvents. As shown in FIG. 1A, in the first stage, an ethanol solution of tetrabutylammonium bromide is used as a titrant with an anhydrous ethanol as a solvent, or an isopropanol solution of potassium hydroxide (KOH) is used as a titrant with a methanol as solvent to titrate nitric acid contained in an acid mixture. As shown in FIG. 1B, in the second stage, a sodium hydroxide aqueous solution is used as a titrant with a saturated sodium chloride solution as a solvent to titrate phosphoric acid and glacial acetic acid contained in the acid mixture. The titration process generates two end points, of which the first end point is associated with nitric acid and the first hydrogen ion of phosphoric acid and the second end point is associated with the second hydrogen ion of phosphoric acid and acetic acid. Through an automatic computation process, the contents of the three acids can be obtained. These conventional approaches are carried out with a two-stage process and computation and the titration operation is complicated and tedious and requires various titrants and solvents, which would lead to increased uncertainty and thereby reduce the accuracy and repeatability of the result of titration.